In the prior art, hammer drills originally had only forwardly rotated motors and therefore the hammering mechanisms could operate at will whenever the selector was set for the hammer mode. However, with the use of reversing switches, which are desirable for the drilling operation, it became necessary to instruct the operator not to use reverse during hammering. Such a warning is not a solution, but unfortunately the operator had no restraint against operating the tool in reverse during hammering. Manual biasing means present a partial solution, but can be defeated by an operator's manual override.